1. Field of the Invention
This invention relates to an image forming apparatus such as a photocopier and a printer and, more particularly, to a charging apparatus for removing charges on recording media upon application of charges to the recording media to which toner is transferred.
2. Description of Related Art
Image forming apparatuses such as printers used for output apparatuses for computers, facsimile machines, and photocopiers for forming images with image data read from image scanners are generally, widely used recently.
With respect to image forming men, for example, various kinds such as thermal melting type, thermal sublimation type, thermal transfer type, inkjet type, and electrophotographic type also have been developed in accordance with respective applications as shown in FIG. 8, a full color electrophotography type image forming apparatus using four colors, cyan, yellow, magenta, black in which four process stations 1a, 1b, 1c, 1d serving as image forming portions for different colors are arranged, inter alia, has been proposed as a high speed color image forming system (namely; an in-line system).
FIG. 8 is a background art of the invention. In FIG. 8, process stations 1a and 1d include photosensitive drums 2a to 2d as image carriers, and latent images are formed on the surface of the photosensitive drums 2a to 2d upon reception of exposure based on image information by means of exposing apparatus 4a to 4d such as, e.g., LED (light emitting device) or laser diode, after the surface is charged evenly with primary charging apparatuses 3a to 3d. The latent images are developed as toner images upon attachment of toners in respective colors by developing apparatuses 5a to 5d. 
The process stations 1a to 1d are detachably attached to the image forming apparatus, body as process cartridges. The respective process cartridges have a structure in which the photosensitive drums 2a to 2d, the primary charging apparatuses 3a to 3d, the developing apparatuses 5a to 5d, and cleaning means 6a to 6d are formed in a united body, respectively.
A transfer material S as a recording material contained in a feeding cassette 15 is fed out to the interior of the image forming apparatus body by a feeding roller 16, is conveyed with a register roller pair 17, and then, is conveyed as carried as attracted electrostatically with a transfer conveyance belt 7 serving as a transfer material carrier and with an attracting roller 12 to which an attracting bias voltage of positive polarity is given by means of an attracting bias power supply 13.
The transfer conveyance belt 7 is tensioned with four rollers, a drive roller 8, an attracting facing roller 9, and a pair of tension rollers 10, 11. The respective color process stations 1a, 1b, 1c, 1d for cyan, yellow, magenta, and black are disposed substantially vertically with respect to the surface of the transfer conveyance belt 7 in order from the upstream side along the moving direction (arrow a direction in FIG. 8) of the transfer conveyance belt 7.
The transfer material S attracted from the transfer conveyance belt 7 passes sequentially through the respective color process stations 1a to 1d, thereby electrostatically transferring sequentially the toner images carried on the photosensitive drum is 2a to 2d. Those toner images are then subject to heat and pressure at a fixing apparatus 18 to be settled on the transfer material S as to form permanent images.
With the apparatus thus described above, however, the transfer material S made of paper or synthetic resin as dielectric is built with charges in a large point because gazing through the transfer process four times when passing through the four process stations 1a to 1d. 
Therefore, separation discharging phenomena occur at a separation portion formed on the peripheral surface of the drive roller 8 from the transfer conveyance belt 7, so that a phenomenon that the toner images transferred onto the transfer material S are disordered according to discharging patterns may occur.
Particularly, such phenomena occur outstandingly where the transfer material S has a high resistance and where the image is formed at an environment of lower temperature and low humidity that easily subjects to static building, or where, in a double side image formation, an image on a second side is formed on a transfer material S in a dried state after the material passes through the fixing process at the first side image formation.
It is therefore general to take resorts to remove charges on the transfer material S by a wire type corona charger 51 when the transfer material S separates from the transfer conveyance belt 7, where the corona charger 51 as shown in FIG. 9 is disposed between the transfer conveyance belt 7 and the fixing apparatus 18 as shown in FIG. 8, thereby canceling image defects upon prevention of discharges at separations.
The corona discharge methods are various, and in general, the corona charger 51 as shown in FIG. 9 is used for obtaining stable discharges. The corona charger 51 shown in FIG. 9 has a tungsten wire 52 having an outer diameter of 60 micron meters shielded with a shield panel 53 having a shape of a rectangular letter U cross section, and a direct current high voltage power supply 54 is connected between the shield plate 53 and the tungsten wire 52.
Such a wire type corona charger 51 can discharge stably and have a feature that good charge removal effects can be obtained easily, but the tungsten wire 52 tends to be readily cut during cleaning when the tungsten wire becomes dirty or during jamming recovery of the transfer material S, so that there is hardship in duration and safety aspect.
On the other hand, a charger having a needle shaped electrode, although not having rifle of wire cutting likewise in the corona charger 51, hardly render stable the charge removal effects because discharges are unstable due to abrupt rising of the current around the discharging threshold, and there are problems requiring some complicated structure to make stable the corona discharge current such that a grid is used at the opening of the charger or that independent resistors are provided where plural needle shaped electrodes are separated electrically from each other.